Diana’s secret affair with heart surgeon Hasnat Khan ended when they both realised that they couldn’t live a ‘normal’ life, says the film’s screenwriter, Stephen Jeffreys.

‘I think Diana was very attracted to Hasnat because of what he did. He had wonderful surgeon’s hands and eyes and she was tremendously excited to be near someone who was doing good work.

'However, in order to do that good work he couldn’t possibly be in the eye of a media storm.

‘When you are doing 12-hour-long heart operations you can’t come out of that and then have a her towards him meant they couldn’t stay together.

'I think that it was a relationship, which I’m sure we’ve all experienced, where you go on longer with it than rationally you should.

‘You kind of know it’s not going to work and there are clear reasons why, but the heart overrules the head.’

Jeffreys, who based his script on journalist Kate Snell’s book, Diana: Her Last Love, and his own extensive research, believes there was a chance that, had she lived, Diana and Khan would have got back together.

‘There was an escape plan that they would go to South Africa or Australia when the boys were older. But in the end I think it was a love affair that was defeated by practicalities.’

Jeffreys’ screenplay centres on the relationship between Khan and Diana. Her more publicised romance with Dodi Fayed, who died in the same Paris car crash that killed her on August 31, 1997, only features briefly at the end of the film.

‘Our view is, that relationship is less important – it was a summer romance; having looked at what she said, you feel that it was a bit of fun for her.’

'I felt Diana's presence during the film. I asked her if I could carry on - and she gave me permission': An extraordinary interview with Diana star Naomi Watts

The actress is under huge pressure to deliver a sensitive portrait of Diana in a biopic that focuses on the secret romance that the late Princess of Wales had with heart surgeon Hasnat Khan in the last two years of her life

'There were definitely moments when I felt Diana’s presence – I dreamed about her a lot, too, and that’s a first,’ says Naomi Watts. ‘I kept wondering to myself: “Would she have liked it?”

‘So I found myself constantly asking for her permission to carry on. I had saturated myself with Diana and her life and I felt this enormous responsibility of playing this iconic woman.

'It felt like I was spending a lot of time with her. There was one particular moment when I felt her permission was granted. That won’t sound right in print, I know.’

Watts is under huge pressure to deliver a sensitive portrait of Diana in a biopic that focuses on the secret romance that the late Princess of Wales had with heart surgeon Hasnat Khan in the last two years of her life.

The actress lived across the road from Kensington Palace while filming Diana, surrounded by reminders of the Princess, and she constantly fretted about how Diana’s family – especially her sons, William and Harry – would feel about the film.

What makes it especially problematic is that very little is known about the relationship, as Khan has never spoken about it publicly, and the film-makers have had to imagine what went on while they were together. Critics are already concerned about how the film will portray the much-loved Princess and whether it is tactless to invent her thoughts and feelings.

Watts says: ‘There was a lot of hesitation on my part before I agreed to do it. Obviously I was taking on one of the most famous women of my time and an awful lot of pressure comes with that.

'You want to get it right, and everybody is going to have an opinion on the film and how she should be portrayed. It’s very daunting.

‘But then I also knew that the exciting roles, the best roles, come with a risk. In the end I decided that I couldn’t not do it.

'This was a story that had to be told – it’s an important story – but it was definitely the hardest thing I’ve ever done. It’s as close as I ever want to get to a character.

‘I lived in a house in Kensington and at 5.30 in the morning on my first day there, in my jet-lagged state, I went for a run not really knowing where I was going. 'All of a sudden I looked up and I was at the gates of Kensington Palace – that felt very eerie,’ she says.

‘I think I became a little obsessed. I felt very connected to her. I was very relieved to finish and leave her behind. But now that the film is ready to come out all of those feelings have come back to me again.’

Watts has played challenging roles before – not least in an Oscar-nominated performance in The Impossible as a young mother trying to find her family after the Asian tsunami – but becoming Diana was different.

One of Watts’s early concerns was that she didn’t look enough like her, so each day before filming started she would spend an hour having a prosthesis placed on her nose to make it look more like Diana’s and burying her own locks under a variety of wigs.

‘I’m sure if someone placed an ad and said, “We’re looking for the best Diana lookalike,” there would be about 7,000 other women before me. But I soon realised it’s not just about matching her physically, it’s about getting inside her – getting the interpretation right.’

For weeks before the cameras started rolling she worked with a voice coach to make sure her voice and pronunciation were exactly right. She also studied Diana’s body language, the way she moved her head, how she smiled.

‘She had a very expressive face,’ she says. ‘And she had that sideways smile we all remember, and those big eyes and a strong, athletic walk.’

Khan is played by Naveen Andrews, best known as Sayid in the hit TV series Lost. Douglas Hodge plays Diana’s butler, Paul Burrell, and Canadian actor Cas Anvar is Dodi Fayed, who features briefly at the end of the film when Diana – her romance with Khan over – makes that ill-fated trip to Paris.

Producer Robert Bernstein says that they had implicit approval from Khan, although he did not meet any of the actors or film-makers.

‘Kate Snell (who wrote the book on which the screenplay is based) has met Hasnat two or three times and they got on very well.

'He trusted her to the extent that he allowed her to meet his family and his friends, and it’s through that relationship that we were able to move forward.

'We’re confident there is a tacit acceptance from his family and Hasnat that what we are doing is OK.

‘The way we are treating the relationship is one of a romantic and tender nature, in keeping with how we feel about Diana and her life.

'It’s a very aspirational, sympathetic portrait and not a voyeuristic one. Certainly our sense of their relationship was that it was very spiritual and sensitive, as well as difficult, obviously. We are handling it very sensitively.’

Bernstein says that they were given coded help from the Palace: ‘We put in a request to film the final scene, where Hasnat Khan comes to the gates of Kensington Palace and lays flowers, and they allowed us to film in front of the gates.

‘In fact, they invited us to film in Kensington Gardens, for a scene where Diana is jogging. 'Certainly the Royals are aware of the film and we were allowed to film in front of the Palace, so make of that what you will; but we haven’t had any direct contact.’The Royal Family have understandably kept their feelings about Diana largely private despite the intense public grief at her death.

Talking exclusively to Event, Watts recounts where she was when she heard the news. ‘I was in Canada filming what ended up being a bad TV movie with Rob Lowe. I was at dinner with him and his wife when the news spread around the restaurant that she had been in a crash. 'By the time we got back to the hotel, Diana had died.

‘I remember being quite traumatised by it. I grew up in England and like everyone watched the Royal wedding. And later when I was in Australia or working in America she was still very present in all of our lives, on the television and in the newspapers. It was all so shocking.’

Before filming began, Watts immersed herself in the role – reading biographies to research Diana’s life and playing old TV footage of her – especially Diana’s famous 1995 Panorama interview with journalist Martin Bashir, during which she laid bare the details of her marriage break-up and the pressures that came with marrying into the Royal Family. ‘I found myself listening to the Bashir interview all the time – when I would jog in the park or last thing at night before going to bed.’ She also met some of Diana’s friends, although won’t reveal their names.‘It was very useful meeting her old friends as it meant I could pick up little details, little stories and make sure I was doing it right. 'The people I met who knew her were unanimous in their praise of her extraordinary sense of humour and her cheekiness. 'They also all said she had quite a rebellious streak, which I think is something I always admire.

‘What also came through was that she was someone who was wildly charismatic in the way that we know someone like Bill Clinton is. 'She would walk into a room and all eyes were on her, and it’s not just because of the level of fame – there was an incredible charm that went with it.

‘I did more research for this role than I’ve ever done before. The problem became that there was so much information out there – books, press articles, stuff on the internet, documentaries – it often became contradictory, which I think goes to show what a complex person she was. I just had to trust my instincts.’

Diana first met Khan in 1995 at the Royal Brompton Hospital where she had gone to visit a friend recovering from surgery.

In the inquest into Diana’s death, Khan said that their relationship began in the late summer of that year and that at one point they had talked of marrying.

‘I think she felt she could trust Hasnat and that’s why their relationship escalated so quickly. 'So many people had let her down in life, people that she felt that she could trust didn’t end up being there for her. They both went to great lengths to keep their relationship private and so for her I think it felt much more authentic.’

In the film, Hasnat encourages Diana to become more involved with the charitable causes she believed in, particularly highlighting the carnage caused by anti-personnel landmines.

‘I think the reason the relationship worked was because it was based on healing others as well as themselves.’ She will not be drawn on whether she feels Diana was let down by the Royal Family after her divorce from Charles in 1996. ‘That’s not for me to judge. I’m just an actor interpreting a character. Who cares about my opinions?’

Watts is a monarchist and very proud of her British heritage.

‘I’m English. I was born here and I lived here until I was 14. And being back in England filming Diana was special for me.’

She can empathise with Diana’s sons as she, too, lost a parent when very young. Her father Peter, a sound engineer with rock band Pink Floyd, died of a suspected drugs overdose in 1976 when Naomi was just eight years old.

‘I’ve thought about her sons, of course, and it’s haunting that they lost their mother because I know what it means to lose a parent so young. ‘I’m always thinking about those boys – I can’t imagine growing up without a mother. Of course it was in my mind when I made the decision to do the film; how would they feel?

'I would hate to upset them. I hope they feel good about it.

'It’s a piece of history that we are all interested in and at some point the story had to be told.’

On set, Watts wore replicas of dresses made famous by Diana and one, a blue mini-dress by designer Jacques Azagury, that was the real thing, loaned to the production by its owner.

‘The Azagury was completely unaltered, and wearing that was quite eerie. 'I was surprised by how short she wore it – it was quite risqué and I’m shorter than her. There were a few unsettling moments like that.’Ultimately, Watts hopes her portrayal of the Princess will give audiences a chance to see what a complex real woman she was.

‘There were the newspaper stories where she was this shiny, happy Royal Princess and then later there were the stories and books about her issues.

‘I think she was fascinating with a lot of contradictions and that’s very human – all of us are like that – and I hope the movie gets that across. 'She was multi-layered, she was a great mother to two boys, she was very funny, she was very strong and she did an incredible amount of good work, not least with landmines. ‘She was just fighting for her happiness. And I think there were moments – like the time she spent with Hasnat – when she achieved it. 'She was not unlike a lot of women in my own life who are strong, intelligent, witty, complicated. She embodied all of those things.’